Boiling Water Reactors (BWR) designed for power generation utilize fuel assemblies arranged inside vertical channels through which water coolant is injected. Each of the fuel assemblies consist of a plurality of vertical rods arrayed within the said vertical channels. The said rods are sealed cylindrical tubes inside which ceramic pellets of fissionable material, e.g. uranium oxide, are stacked. The water flows upward in the channels and removes the heat generated in the pellets by the fissioning of the heavy isotopes. In addition to its cooling function, the water serves a neutron moderator. The said moderator function is achieved as the neutrons produced in the fission process collide with the hydrogen atoms in the water molecules and slow down to lower energies which increase the probability of inducing further fission reactions and the fission chain reaction is sustained.
In boiling water reactors, the water is allowed to boil as it travels up in the fuel assembly channel. The density of water is reduced by the boiling process and the moderating function is adversely affected particularly in the upper portion of the fuel assembly, where the fuel-to-moderator ratio becomes higher than optimally desired. This problem was mitigated by introducing one or more water rods or channels, henceforth called water channels, A water channel is a hollow tube or conduit extending vertically along the fuel rods, and through which part of the water flows without boiling. Thus, the amount of water available for the neutron moderating function is increased. The said improvement in the moderation function comes at the expense of reducing the amount of water available for the cooling function. This is obviated by the fact that the total amount of water flowing through a given fuel assembly is limited. The diversion of part of the coolant water to water channels may result in reducing the maximum power that can be safely produced by a fuel assembly containing such water channels, and imposes economically undesirable limitations on the reactor core design and reactor operations.
The amount of coolant diverted to flow through the water channel is determined by the need to maintain its liquid state (no boiling) so that the hydrogen atom density remains high for the neutron moderation function. As the neutrons give off their kinetic energy to the moderator, the moderator enthalpy (and temperature) rises. An additional source of energy deposition in the moderator is the absorption of gamma rays produced in the fission process. As moderator boiling in the water channel defeats its purpose, there must be sufficient flow through the channel such that the neutron and gamma energy deposited in the water channel is less than the water subcooling at the inlet, where the subcooling is defined as the enthalpy of saturated water less the enthalpy of the entering water. The amount of water diverted to flow in the water channel to avoid boiling can be as high as .apprxeq.5% of the total flow.
Attempts were made in the prior art to save the water channel flow for cooling function by adding a flow path starting at the top of the water channel and guiding the water flow in reverse to be re-injected into the cooling stream at a lower elevation. Such attempts were not successful because the water column between the re-injection point and the top of the water channel structure becomes unstable, where steam can be entrapped in that space defeating the purpose of the water channel. Reinjection water channels have been abandoned in practice in favor of once-through tubes.
The water channels of the prior art can be useful for enhancing the stability of BWR cores as explained qualitatively hereunder. For a stable operation of a BWR, a small perturbation of the flow rate entering the bottom of fuel assemblies must not grow into larger fluctuations. The mechanism for such possible growth depends on various time delay and feedback strength of various thermal-hydraulic and neutronic interactions. Consider a small sinusoidal inlet flow perturbation The initial small increase in inlet flow will result in decreasing the voiding of the cooling water at higher elevations. The said decrease in voiding travels upwards along the fuel rods creating the well-known density wave. Thus, the effective increase in the total pressure drop across the assembly is delayed by the time it takes the said density wave to travel from the bottom single-phase region to the two-phase region in the top section of the assembly. If the said delay time is equal to half the period of the initial sinusoidal perturbation, an oscillation of such frequency can be potentially amplified, as the said increase in the flow resistance coincides and reinforces the flow decrease of the second half of the inlet flow perturbation. Such oscillations are likely to grow if the two-phase pressure drop occurring in the top section of the assembly (the delayed component) is large compared with the single-phase pressure drop occurring in the bottom section of the assembly. An additional feedback mechanism is provided by the fission power response to said density wave. The density wave is equivalent to a change in the moderator density, which directly changes the nuclear reactivity. The change in said nuclear reactivity produces a change in the fission power deposited inside the fuel rods with a small time delay. The said change of fission power is released into the cooling flow with a time delay associated with the heat conduction in the fuel rods. The result of this energy transfer response to the coolant is a corresponding change in voiding of the coolant and pressure drop that reinforces the initial inlet flow perturbation. For such neutronic feedback to be effectively destabilizing, large void/reactivity coefficient (large reactivity response to change in coolant voiding) is necessary.
The water channels of the prior art influence the stability of density waves in BWR fuel assemblies in two opposing ways:
1. The water channel flow diverts flow from the coolant path, and thus increases the void content and length of the two phase region in the upper section of the assembly, thus increases the two-phase pressure drop relative to the single-phase pressure drop. This effect is destabilizing. An additional secondary effect is the shift of neutron flux distribution to the bottom of the assembly due to the decrease in moderator density in the top of the assembly. The said shift of neutron flux is associated with similar shift in the axial fission power distribution towards the bottom of the assembly. This increase in the bottom power results in coolant voiding at lower elevation, which is a destabilizing effect as explained earlier.
2. The water channel creates a moderator zone that is not affected by the boiling process, and therefore the effect of the boiling of the coolant outside the water channel on the nuclear reactivity is reduced. This reduction in the magnitude of the overall void/reactivity coefficient is stabilizing.
Although the combined effect of the water channel on the density wave stability has been generally favorably, the new water channel concept is far more effective in stabilizing a fuel assembly by influencing different mechanisms that were not affected in the prior art.
It can be concluded that a new design concept for water channels is needed for the purpose of reducing their negative impact on the thermal performance of boiling water reactor fuel assemblies. Also, a new design of water channels that improves the stability performance in new ways is also useful.